1. Field of the Invention
The present invention relates to a catalyst for producing maleic anhydride by oxidizing butane and a process for producing the catalyst.
2. Description of the Related Art
Vanadium and phosphorus-containing compounds are used as catalysts for producing maleic anhydride by oxidizing n-butane with an oxygen-containing gas. The vanadium and phosphorus-containing compounds effective as the catalyst are known to be a crystalline compound of the formula, (VO).sub.2 P.sub.2 O.sub.7. In order to obtain the effective catalyst component, (VO).sub.2 P.sub.2 O.sub.7, usually V.sub.2 O.sub.5 is reduced to V.sub.2 O.sub.4 according to a conventional method, and the V.sub.2 O.sub.4 is reacted with phosphoric acid to prepare (VO).sub.2 H.sub.4 P.sub.2 O.sub.9, and then this compound is thermally decomposed. In order to produce the catalyst precursor, (VO).sub.2 H.sub.4 P.sub.2 O.sub.9, usually V.sub.2 O.sub.5, a reducing agent and phosphoric acid are caused to react in an organic solvent, in particular, an alcohol such as isobutyl alcohol and the like and the end product, (VO).sub.2 H.sub.4 P.sub.2 O.sub.9, is separated as a precipitate, and therefore, it is convenient to isolate and recover the effective catalyst component.
Many attempts have been made to add promoter components to vanadium and phosphorus-containing compounds. For example, examples of addition of promoter components are summarized in Burnett et al., Catalyst Today, 1, 537 (1987), but effects of addition and function and mechanism of promoters are not discussed. Examples of using magnesium compounds and zirconium compounds are not disclosed.
In Japanese Patent Application Laid-Open No. 146,992/78 is disclosed a process in which a catalyst precursor is formed by reacting a vanadium compound with phosphoric acid in the presence of an acid stronger than phosphoric acid. In the specification, rare earth elements and 15 other elements including zirconium are mentioned as examples of promoters. However, nothing is specifically explained with regard to the effects of zirconium.
In Japanese Patent Application Laid-Open No. 13,483/79 is proposed a process in which a catalyst precursor is synthesized by subjecting a pentavalent vanadium compound to reduction with a powdered metal or a colloidal metal and at the same time to hydrothermal synthesis. In the specification, magnesium is mentioned as an example of a metal reducing agent, but this method is not preferred since expensive metals are used as a reducing agent. Further, other than the metal reducing agents, 14 elements including zirconium and alkaline earth and alkaline metals are mentioned as promoters. However, nothing is mentioned about the effect resulting from the addition of zirconium.
In Japanese Patent Application Laid-Open No. 30114/79 is disclosed a process in which 23 promoters including magnesium are incorporated in vanadium-phosphorus system catalysts by impregnation. However, according to the working examples, the catalyst containing vanadium and phosphorus can give maleic anhydride in only 49% yield and therefore, is not sufficiently effective for industrial purpose.
In Japanese Patent Application Laid-Open No. 24,643/82 is disclosed an annular catalyst consisting of vanadium and phosphorus compounds. In the specification, 48 elements including magnesium and zirconium are mentioned as examples of metals to be incorporated. There are however no specific explanations on the effects of addition of the metallic elements.
In Japanese Patent Application Laid-Open No. 114,735/83 is described a process for activating vanadium and phosphorus-containing catalysts with oxygen and a reducing gas. In the specification, alkali metals, alkaline earth metals, rare earths and 13 other elements including zirconium are mentioned as possible promoter components. However, nothing is specifically explained with regard to the effect of magnesium, other alkaline earth metals and zirconium.
In Japanese Patent Application Laid-Open No. 12,759/84 is proposed a method wherein a catalyst precursor is recovered by adding water to an organic slurry containing the precursor resulting in separation into two phases. As examples of promoter components, mention is made of alkali metals, alkaline earth metals, lanthanides, an other 23 elements including zirconium. However, nothing is specifically described with regard to the promoter effects of magnesium, other alkaline earth metals and zirconium.
In Japanese Patent Application Laid-Open No. 55,350/84 is disclosed a method for spray drying a catalyst precursor after it has been pulverized and densified. In the specification, mention is made of promoter components such as alkali metals, alkaline earth metals, rare earths, and 24 other elements including zirconium. However, there are contained no specific descriptions with regard to the promoter effects of magnesium, other alkaline earth metals and zirconium.
In Japanese Patent Application Laid-Open No. 145046/84 is described a process for preparing a catalyst by mixing a first component, a crystalline vanadium and phosphorus-containing compound; a second component, an aqueous vanadyl phosphate; a third component, a compound of at least one element selected from magnesium, calcium, strontium and barium; and a fourth component, silica and drying the mixture. However, according to the present inventors' study, the catalyst prepared in such a procedure is not suitable for industrial production due to the low yield of maleic anhydride.
In Japanese Patent Application Laid-Open Nos. 227,835/85 and 227,836/85 is described a process for preparing catalysts in which a vanadium and phosphorus-containing catalyst precursor is admixed with a paste of hydrated zirconium hydroxide. However, according to studies by the present inventors, catalysts prepared in accordance with the process give no significant improvements in the yields of maleic anhydride.
Zirconium is mentioned as a possible promoter component in Japanese Patent Application Laid-Open Nos. 71,641/82; 122,944/82; 130,552/82; 132,550/82; 84,045/83; 87,049/84 and 181,540/86. In the specifications of these patents, nothing is explained on the effects of zirconium.